Atomic-scale study of the amorphous-to-crystalline phase transition mechanism in GeTe thin films

The underlying mechanism driving the structural amorphous-to-crystalline transition in Group VI chalcogenides is still a matter of debate even in the simplest GeTe system. We exploit the extreme sensitivity of 57Fe emission Mössbauer spectroscopy, following dilute implantation of 57Mn (T½ = 1.5 min) at ISOLDE/CERN, to study the electronic charge distribution in the immediate vicinity of the 57Fe probe substituting Ge (FeGe), and to interrogate the local environment of FeGe over the amorphous-crystalline phase transition in GeTe thin films. Our results show that the local structure of as-sputtered amorphous GeTe is a combination of tetrahedral and defect-octahedral sites. The main effect of the crystallization is the conversion from tetrahedral to defect-free octahedral sites. We discover that only the tetrahedral fraction in amorphous GeTe participates to the change of the FeGe-Te chemical bonds, with a net electronic charge density transfer of ~ 1.6 e/a0 between FeGe and neighboring Te atoms. This charge transfer accounts for a lowering of the covalent character during crystallization. The results are corroborated by theoretical calculations within the framework of density functional theory. The observed atomic-scale chemical-structural changes are directly connected to the macroscopic phase transition and resistivity switch of GeTe thin films.
This work was supported by the European Union Seventh Framework through ENSAR (Contract No. 262010). R. Fallica, C. Wiemer, and M. Longo acknowledge the SYNAPSE project (“SYnthesis and functionality of chalcogenide NAnostructures for PhaSE change memories”), which received funding from the European Union Seventh Framework Programme (FP7/2007–2013), under grant agreement n° 310339. H. P. Gunnlaugsson acknowledges support from the Fund for Scientific Research-Flanders and the KU Leuven BOF (SF/14/013, CREA/14/13, and STRT/14/002). K. Bharuth-Ram, H. Masenda, D. Naidoo, and M. Ncube acknowledge support from the South African National Research Foundation and the Department of Science and Technology. T. E. Mølholt, H. P. Gislason, and S. Ólafsson acknowledge support from the Icelandic Research Fund (Grant No. 110017021-23). The authors acknowledge Numonyx for the GeTe samples and Dr. Enrico Varesi (now at Micron), Dr. Davide Erbetta (now at STMicroelectronics) and Dr. Roberto Bez (now at LFoundry) for scientific discussion.